1. Technical Field
The present invention relates to ink jet printers. More particularly, the present invention relates to a method and apparatus for providing consistent, high quality, multi-color ink jet printing having variable print density on any of various media.
2. Description of the Prior Art
Since the introduction of ink jet printing technology, the accepted solutions to the problems of leathering, dot gain, recording medium cockle and curl, and color bleed with single-part inks have included optimizing the formulation to the extent possible, minimizing the rate at which ink is printed on the recording medium, and adding energy to the ink either before or after printing to accomplish a complete or partial phase change. Such phase change includes both liquid-solid and liquid-vapor transitions, as well as significant changes in solubility and viscosity. In both of the foregoing methods, the print quality ("PQ") dependence on medium surface characteristics remains the same because the desired ink phase change does not occur rapidly enough to prevent some colorant migration on the surface of the recording medium, usually paper or transparency film. Thus, printing characteristic and resulting print quality varies from medium to medium.
So called tiling algorithms have been used to minimize the amount of ink applied to the medium and thus avoid somewhat color-to-color bleed and paper cockle and curl. Such tiling algorithms control the amount of ink applied during printing to produce a depleted matrix on each pass of a scanning printhead. A solid area fill is thereby eventually built up only after several passes of the printhead. In this way, the medium is not saturated during any one printing pass, and the medium is allowed to dry partially during the interval between printing passes.
However, such tiling algorithms slow the printing process, resulting in a significant throughput penalty and cannot easily be applied to page-wide array printing, which is adapted to single-pass, multicolor printing. The throughput penalty of tiling schemes seriously degrades printer performance when the printer is used for color printing, where several printing passes must be made for each of several colors. Such performance degradation is especially pronounced when the throughput achieved for color printing is compared to the throughput achieved for black-only printing: usually a 3:1 or 4:1 time penalty is realized (color print time: black print time).
The addition of energy to the printing process by heating the recording medium immediately before and/or during printing increases the rate of evaporation of the ink vehicle from the medium surface, thereby accelerating the rate at which ink applied to the medium dries at the medium surface. When paper is used as the recording medium, this ink vehicle evaporation limits penetration of the ink into the paper fibers and also limits capillary action on the surface of the paper. These two effects reduce cockle and color bleed. However, energy addition must be adapted to the type of recording medium (plain paper, special paper, and overhead transparency film) and to print density. Heaters generally require a fan to remove ink vapor, and these components together significantly increase the cost and complexity of desktop printers.
In another method involving the addition of heat to the printing process, a hot-melt (solid) ink is delivered to the recording medium in a liquid state. The ink freezes upon coming into contact with the recording medium surface, thereby immobilizing the colorant component of the ink on the surface of the recording medium. The addition of solid ink delivery systems and the associated thermal management required by such systems significantly increases the cost and complexity of the printer, as well as increasing the demands made on the materials used in fabricating the print head, which must typically operate at 150.degree.-200.degree. C.
In either case above, the use of excessive heating causes undesirable structural changes in the medium, for example dimensional changes, browning, and embrittlement due to water loss in paper media, or do to melting in polymer based media, such as transparency films. Additionally, if the ink vehicle is evaporated too rapidly, the various colorants used in the ink can precipitate on the medium surface, resulting in browning of the printed region and poor adhesion of the colorant to the medium surface.
Reactive chemistry provides another energy source for effecting ink phase change that may be used in color printing. This approach mitigates some of the foregoing problems by introducing anionic/cationic reactions among the several colorants applied to the recording medium. Thus, the energy source for phase change is provided in the form of chemical energy in a binary ink system where one of the two components of the system contains a colorant and the other a fixer. Such two-part ink system undergoes a chemical reaction on the surface of the recording medium.
Typically, a first component of the binary system is applied contemporaneously with, or a short interval before, a second component of the binary system. Both pre- and post- treatments have been considered in the art. When the two components are combined on the recording medium, a chemical reaction takes place that immobilizes the ink. The various means taught in the art for applying such multi-part inks have included wetted rollers and wipers on the unprinted media, spray heads and misting devices, and thermal ink jet printheads.
A number of methods have been proposed to immobilize the colorant in situ on the paper surface. Such known methods include metal ion chelating, anionic/cationic reactions, and polymeric reactions. See, for example U.S. Pat. No. 4,694,302, Hackleman et al, Reactive Ink-Jet Printing, which discloses a method for increasing the water fastness of an ink-jet ink by combining the ink with a reactive species, either present in the paper itself, or separately applied to the paper.
The use of multi-component inks in thermal ink jet printers has been the subject of only limited investigation by researchers. To the extent that such research has been pursued, it has been narrowly focused on solving the color bleed problem. Thus, multi-component inks have not been investigated, proposed, or adapted for use in ink jet printers to eliminate the coupled problems of wet and dry paper cockle, color bleed, slow dry time (i.e. the time that elapses after printing a region with any color before the printed region is sufficiently dry that it cannot be smeared), and allow media-independent high quality single-pass color printing. Nor have such inks been suggested as a viable alternative to the commonly accepted use of both multi-pass tiling algorithms and the application of thermal energy to the ink jet printing process.